Process and apparatus to subdivide objects

ABSTRACT

A process and an apparatus used to subdivide objects ( 4 ) into slices, in particular to manufacture wafers from semi-conducting monocrystalline blocks, wherein these blocks are subdivided into slices with the help of at least one cutting tool ( 3 ), and under the influence of an etchant. Prior to completely cutting and subdividing the object ( 4 ) (ingot) into individual slices (wafers), a separating foil ( 12 ) is introduced into each cut slit ( 9 ) to separate the slices. This efficiently prevents the resultant wafers from sticking together. The individual separation of the wafers is thus accomplished right when the cutting process takes place so that the necessary number of process steps to manufacture silicon wafers is reduced.

BACKGROUND

[0001] This invention pertains to a process and an apparatus tosubdivide objects, in particular to produce wafers from semi-conducting,monocrystalline blocks, wherein these blocks are subdivided into sliceswith the help of at least one cutting tool and under the influence of anetchant.

[0002] The apparatus has a cutter arrangement that has at least onecutting tool and an etchant feed device.

[0003] In the solar cell and chip industry, very thin wafers arenecessary that have very good electronic properties. Currentmanufacturing processes deal with the mechanical abrading of an objectto be cut containing the monocrystalline block (ingot). What is mostcommonly used for this is the so-called multi-wire slurry saw (MWSS).This mechanical process results in high surface damages to the silicon,making subsequent etching and cleaning steps necessary, and can lead toincreased breaks. In addition, the saw losses are high and the minimumattainable wafer thickness is limited to approximately 200 μm.

[0004] Another so-called “stream etching” process is known in which ataut wire is used as the cutting tool. Etchant flows along this wire.The wire and the block to be subdivided are connected to a currentsource so that the cutting process occurs in a non-contact fashionthrough electrochemical etching. This process results in wafers withplanar, practically undamaged surfaces, with the breakage rate and thecutting losses being minimized, and allowing the manufacture of verythin wafers. The problem with this process is that these thin wafers canstick together by adhesion, making it very difficult to separate them.

SUMMARY

[0005] The object of this invention is to provide a process and anapparatus that allows very thin wafers or similar sliced objects to bemanufactured while safely and reliably separating them.

[0006] To accomplish this object, the process according to the inventionproposes to introduce into each slit cut a foil to separate the slicesprior to the object (ingot) being completely cut and subdivided intoindividual slices. It is suggested that the apparatus used for thispurpose be provided with one or more foils to be introduced into therespective slit cut, which is produced by the cutting tool or cuttingtools, between adjacent slices in the object to be subdivided. Thesefoils are then used to separate the adjacent slices.

[0007] This efficiently prevents the resultant wafers from stickingtogether. The individual separation of the wafers is performed rightwhen the cutting process is performed so that the necessary number ofprocess steps in the manufacturing of silicon wafers is reduced.

[0008] It is advantageous for the apparatus to have a foil carrier forone or more separating foils to separate adjacent slices. This carrierhas a device to hold the separating foil(s) taught inside the slits cut.

[0009] If a foil carrier is provided for a number of adjacent parallelfoils, it is preferable to also have an adjustable distance maintainer.

[0010] This allows the precise setting and adjustment of theparallelness of the foils if a number of wafers are produced at the sametime.

[0011] The separating foil(s) are formed, in particular, of a materialthat is resistant to the etchant used, the temperature present duringthe process as well as any existing electrical potentials.

[0012] It is preferred that the separating foil be made ofpolytetrafluoroethylene (PTFE).

[0013] For the electrochemical cutting of silicon using potassiumhydroxide, thin PTFE foils are particularly well suited since they havea very low coefficient of friction, are not wetted by potassiumhydroxide and are resistant to potassium hydroxide.

[0014] In order to ensure an improved mechanical separation ofelectrolyte and foils, the separating foil can have at least oneexterior side with a profiled or structured surface, in particular witha microstructured surface. If necessary, it can have a reticularstructure.

[0015] Additional embodiments of the invention are discussed below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The preferred embodiment of the invention and the details thereofare explained in more detail with the help of the drawings.

[0017] The lone FIGURE shows schematically:

[0018] A perspective view of a cutter to subdivide into slicesmonocrystalline blocks, in particular, using cutting tools formed bywires.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0019] A cutter 1 shown in the FIGURE is used to subdivide, particularlyinto slices, semi-conducting objects. In the preferred embodiment, thecutter 1 has a number of adjacent parallel wires 2 as the cutting tools3. Also, separators 12, which are separate from the cutting tools 3, areprovided that are made of foil.

[0020] In the cutting process, the wires are placed in the directvicinity of the surface of the object 4 to be subdivided. The wires 2are held taught and are wetted with an etchant fluid that flows downthese wires 2 and also wets the surface of the object 4 in the directvicinity of each of the wires.

[0021] The object 4 to be subdivided is provided with a large surfacecontact electrode 7 opposite the side being cut 6. This contactelectrode 7 is connected to a DC power supply 8. The electricallyconducting wires 2 are connected to the other side of this power supply.It is also possible to use an AC power supply.

[0022] The etching is started locally by activating the DC power supply.The cutting process then proceeds through electrochemical etching.

[0023] In the electrochemical etching, a cut slit 9 arises, wherein thedistance between the wires 2 is held constant at right angles to theobject 4 to be subdivided by moving the object 4 in the direction ofarrow Pf 2. This enlarges the cut slit 9 in the cutting directionaccording to arrow Pf 1. To maintain constant distance between the wire2 and the object 4, the relative motion is attained by moving the object4 in the direction of arrow Pf 2.

[0024] As shown schematically in the FIGURE, adjacent wires 2 next toone another are provided as the cutting tools, and foils act as separateseparators 12 that are some distance away from the cutting tool 3(wire). During the cutting process, these separator foils are broughtapproximately centered into the respective slit cut and are at enough ofa distance from the wire 2 that they are not wetted with etchant.

[0025] Polytetrafluoroethylene can be used as the preferred material forthe separator foils because these types of foils have a very lowcoefficient of friction, are not wetted by, for example, potassiumhydroxide as etchant fluid and are also resistant to these etchingfluids.

[0026] In order to prevent the foil separators 12 from sticking to thecut surfaces of the object 4 near the cut slit 9, the foils can have oneor both external sides with a profiled or structured surface, inparticular a microstructured surface. If necessary, it can also have areticular structure in order to ensure an improved mechanical separationof electrolyte and foil.

[0027] Instead of a wire, a strip can also be used as cutting tool 3, inparticular a foil strip. In the process, the foil strip can serve bothto separate the two adjacent slices and as a cutting tool. This preventsthe thin slices from sticking together.

[0028] It is useful to have this foil extend at least over a largeportion of the cut surface of the object to be subdivided in order toprevent adhesion throughout practically the entire cut zone.

1. A process to subdivide objects (4) for production of wafers fromsemi-conducting, monocrystalline blocks, comprising subdividing theblocks into slices with at least one cutting tool (3) and an etchant,and introducing a separating foil (12) into each slit cut (9) toseparate the slices prior to complete cutting and subdivision of theobject (4) into the individual slices.
 2. The process according to claim1, further comprising moving the separating foil (12) into the slit cut(9) outside of an etchant wetting zone without touching the subdividedobject (4).
 3. The process according to claim 1, wherein the separatingfoil (12) moves approximately centered in the respective slit cut (9) ofthe object (4).
 4. The process according to claim 1, wherein theseparating foil (12) moves at a distance from the respective cuttingtool (3).
 5. An apparatus to subdivide objects (4) into slices, tomanufacture wafers from semi-conducting monocrystalline blocks,comprising a cutter (1) having at least one cutting tool (3) and anetchant feed device, and one or more separating foils (12) provided toseparate adjacent slices, the foils being introduced into each slit cut(9) between adjacent slices in the object (4) to be subdivided.
 6. Theapparatus according to claim 5, further comprising a foil carrier forthe one or more separating foils (12) to separate the adjacent slices,the carrier having a device to hold the separating foils tight inside ofthe slits cut (9).
 7. The apparatus according to claim 6, wherein thefoil carrier has an adjustable distance maintainer for multiple paralleladjacent separating foils (12).
 8. The apparatus according to claim 5,wherein the separating foil (12) is formed of an etchant reiestantmaterial that can withstand a temperature present during processing aswell as any existing electrical potentials.
 9. The apparatus accordingto claim 5, wherein the separating foil (12) is comprised ofpolytetrafluoroethylene (PTFE).
 10. The apparatus according to claim 5,wherein the separating foil (12) has at least one exterior side with atleast one of a profiled surface, structured surface, a microstructuredsurface and a reticular structure.
 11. The apparatus according to claim5, wherein the separating foil (12) is adapted to extend at least over alarge portion of a respective cut surface in the block (4) to besubdivided.
 12. The apparatus according to claim 5, wherein a wire (2)is provided as the cutting tool (3).
 13. The apparatus according toclaim 5, wherein a strip is provided as the cutting tool (3).
 14. Theapparatus according to claim 13, wherein the strip is a the foil stripthat is used as the cutting tool and extends at least over a largeportion of a cut surface in the object (4) to be subdivided.